@link :section links to a section by its heading, with an alone value (from dev 831455f36fcd)

A section link names the TEXT of a heading: :section alone means the current
file, a value names the file of the same @document that has it. Resolved by
@document at assembly in html and tex; errors for a missing file, a missing
title, a doubled title and a link outside a @document. The Markdown
converter reads a Markdown anchor as a section link to the heading its slug
names. Also from dev: the converter's four silent defects fixed and --chapter
(9ae2c4cd), and the engine's alone-value error text.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

sks/link/link.py

@@ -6,8 +6,25 @@ import klammer_base
 import urllib
 import kutil
 
-def html_link(target, link_text, is_section, no_quotation_marks):
-    # [__link__target__text__]
+# :section (argtype section_link, Andy 2026-10-12) is absent ("false"),
+# written alone ("true": the section is in the current file) or written
+# with a basename (the file that has the section).  A section link is
+# resolved by @document when it assembles its files (sks/document/
+# section_link.h), from this marker: the heading's text and the file, ""
+# meaning the current one, separated by KTSL because either may contain an
+# underscore.
+def section_file(section):
+    """The file a :section value names: "" for the current file, None when
+    the link is not a section link at all."""
+    s = str(section)
+    if s in ("", "false", "False"):
+        return None
+    return "" if s in ("true", "True") else s
+
+def section_marker(title, file):
+    return f"__seclink__{title}__KTSL__{file}__"
+
+def html_link(target, link_text, is_section, no_quotation_marks, file=""):
     if not link_text:
         if is_section:
             link_text = target if no_quotation_marks else f"“{target}”"
@@ -16,14 +33,11 @@ def html_link(target, link_text, is_section, no_quotation_marks):
             link_text = re.sub('/', '/<wbr/>', target)
             link_text = re.sub('/<wbr/>/<wbr/>', '//', link_text)
             link_text = f'<tt>{link_text}</tt>'
-    target = f"[{target}]" if is_section else target
-    #result = f"[__link__{target}__{link_text}__]"
-    #return result
 
-    if target.startswith("http") or ".html" in target:
+    if is_section:
+        url = section_marker(target, file)
+    elif target.startswith("http") or ".html" in target:
         url = target
-    elif is_section:
-        url = f"__sectionlink__{target}__"
     else:
         url = f"^#{target}"
     result = f'<a href="{url}">{link_text}</a>'
@@ -38,7 +52,7 @@ def add_latex_footnote_if_necessary(url, link_text):
         result += f'\\footnote{{\\\rule{{0pt}}{{12pt}}\\texttt{{{escaped_url}}}}}'
     return result
 
-def latex_link(target, link_text, is_section, no_quotation_marks, add_footnote):
+def latex_link(target, link_text, is_section, no_quotation_marks, add_footnote, file=""):
     if target.startswith("http"):
         if link_text and link_text != "URL":
             result = f"#/ \\href{{{target}}}{{{link_text}}} #/"
@@ -48,7 +62,7 @@ def latex_link(target, link_text, is_section, no_quotation_marks, add_footnote):
     elif is_section:
         if not link_text:
             link_text = target if no_quotation_marks else f"``{target}''"
-        result = f"\\hyperlink{{__seclink__{target}__}}{{{link_text}}}"
+        result = f"\\hyperlink{{{section_marker(target, file)}}}{{{link_text}}}"
     else:
         result = f"\\hyperlink{{{target}}}{{{link_text}}}"
     #    if add_footnote:
@@ -63,10 +77,12 @@ class Link(klammer_base.Klammer_base):
         # self.show("Link")
           
     def html(self):
-        return html_link(self.target, self.text, self.section, self.nq)
+        file = section_file(self.section)
+        return html_link(self.target, self.text, file is not None, self.nq, file or "")
 
     def tex(self):
-        return latex_link(self.target, self.text, self.section, self.nq, self.footnote)
+        file = section_file(self.section)
+        return latex_link(self.target, self.text, file is not None, self.nq, self.footnote, file or "")
 
     def txt(self):
         text = f'{self.text} ' if self.text else ''